* Says international criminal justice system should apply (adds Pillay remarks in statement, updates death toll)

By Stephanie Nebehay

GENEVA, July 31 (Reuters) - The top U.N. human rights official said on Thursday she believed Israel was deliberately defying international law in its military offensive in Gaza and that world powers should hold it accountable for possible war crimes.

Israel has attacked homes, schools, hospitals, Gaza's only power plant and U.N. premises in apparent violation of the Geneva Conventions, according to Navi Pillay, U.N. High Commissioner for Human Rights.

"Therefore I would say that they appear to be defying... deliberate defiance of obligations that international law imposes on Israel," Pillay told a news briefing.

"This is why again and again I say we cannot allow impunity, we cannot allow this lack of accountability to go on."

Hamas militants in Gaza have also violated international humanitarian law by firing rockets indiscriminately into Israel, Pillay said.

She also criticised the United States, Israel's main ally, for failing to use its influence with the Jewish state to halt the carnage.

"Many of my remarks have been directed to the United States since they are a party with influence over Israel to do much more to stop the killing, to bring the parties to the negotiating table. I've called also for an end to the (Israeli) blockade (of Gaza)..."

Pillay said she was appalled at Washington consistently voting against resolutions on Israel and the territories in the Human Rights Council, General Assembly and Security Council.

"They have not only provided the heavy weaponry which is now being used by Israel in Gaza but they've also provided almost $1 billion in providing the 'Iron Domes' to protect the Israelis from rocket attacks," she said. "But no such protection has been provided to Gazans against the shelling."

Israeli Prime Minister Benjamin Netanyahu, facing international alarm over a rising civilian death toll in Gaza, said on Thursday he would not accept any ceasefire that stopped Israel completing the destruction of militants' infiltration tunnels.

Gaza officials say at least 1,410 Palestinians, most of them civilians, have been killed and nearly 7,000 wounded. Fifty-six Israeli soldiers have been killed in Gaza clashes and more than 400 wounded. Three civilians have been killed by Palestinian shelling in Israel.

"What we are witnessing (in Gaza) is the killing of entire families, and of children in the street either playing or trying to find safety," Pillay said in a statement.

"Waves and waves of ordinary people continue to flee their homes as the already weak infrastructure in Gaza caves in under the relentless bombardment."

Israel says it is acting to stop the Hamas rocket attacks.

Pillay said she did not expect Israel to investigate properly violations committed during its air strikes and ground assault on Gaza, now in its fourth week.

"But international law is clear that where a state is unable or unwilling to carry out investigations and prosecutions, the international (criminal justice) system applies," she said.

Previous U.N. commissions of inquiry into Israel incursions into Gaza have called for the U.N. Security Council to refer the situation to the prosecutor of the International Criminal Court (ICC), according to Pillay, a former U.N. war crimes judge.

"Accountability and justice cannot be expected to be achieved through domestic proceedings. This is evident from the lack of adequate investigations by Israel and no attempt whatsoever made by the international community to implement the recommendations made by the Gaza fact-finding mission report."

The U.N. Human Rights Council launched an international inquiry last week into violations and crimes that may have been committed by Israel during its latest offensive. (Reporting by Stephanie Nebehay, Editing by Angus MacSwan)

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